1. Field of the Invention
This invention relates to analog-to-digital (A/D) converters. More particularly, this invention relates to A/D converters of the voltage-to-frequency type, commonly designated VFC in this art.
2. Description of the Prior Art
A voltage-to-frequency converter typically produces a train of pulses having a frequency which is proportional to an applied input signal. The speed of a VFC is an important characteristic, and is reflected by the full-scale frequency of the device. Very high speed VFCs (e.g. 20 MHz full scale) are available in modular form, but are quite costly to manufacture. Much less costly VFCs are available in integrated-circuit (IC) form, but have limited speed, e.g. full-scale frequency of 1 MHz.
SUMMARY OF THE INVENTION
In a preferred embodiment of the invention, to be described hereinbelow in detail, there is provided a voltage-to-frequency converter designed for IC application, and having a full-scale output frequency of 30 MHz. Such high speed operation is made possible by the novel architecture of the converter which comprises two cascaded components which are individually known per se. The first component is a triangular-wave VFC (referred to in short as a triwave VFC), and the second component is a device referred to herein as a voltage-slope-to-periodic-function converter (shortened to VSTPF). A conventional triwave converter may have a nominal full-scale output frequency of, say, 1 MHz. That output signal frequency is effectively multiplied by the VSTPF to produce a final full-scale output frequency which might, for example, be in the range of 30 MHz.
The speed of response of a triwave VFC is limited effectively to the period of one cycle. That is, when the input signal changes, that fact is not evidenced in the triwave output frequency until one complete cyclic interval (i.e. one up-and-back variation) of the triwave signal is traversed. However, the triwave output signal actually shows the effect of the changed input signal essentially immediately, because the slope of the triwave output signal is correspondingly altered when the input signal changes. In accordance with the present invention, the changed output signal slope is sensed promptly by the voltage-slope-to-periodic-function converter coupled to the output of the triwave VFC, and is evidenced with very little time delay by a corresponding change in the pulse repetition frequency of its output pulse train. In effect, the VSTPF device extracts advance information from the ramp rate of the triwave output signal. Consequently, the overall speed of operation of the VFC is greatly increased.
Other objects, aspects and advantages of the invention will be pointed out in, or apparent from, the following detailed description of a preferred embodiment of the invention, considered together with the accompanying drawings.